Can someone explain it to me?

Some of the Bosmer, when the time comes, will morph into savage beasts and can never turn back into Bosmer again. They do it as a last resort to defend themselves. One recorded to date is when they killed King Borgas of Skyrim, the last of Ysgammor's line.

Not to be confused with Lord Hircine's Wild Hunt.

A ritualistic hunt with rites and laws that pit the prey against the all-powerful Huntsman and his hounds. Read "Posting of the Hunt".

There is also that other Wild Hunt with Wood Elves and whatnot.